Advanced forming method and structure of local mechanical strained transistor

ABSTRACT

Embodiments of the invention provide a semiconductor fabrication method and a structure for strained transistors. A method comprises forming a stressor layer over a MOS transistor. The stressor layer is selectively etched over the gate electrode, thereby affecting strain conditions within the MOSFET channel region. An NMOS transistor may have a tensile stressor layer, and a PMOS transistor may have compressive stressor layer.

TECHNICAL FIELD

This invention relates generally to semiconductor devices, and, moreparticularly, to methods and structures for controlling strain intransistors to improve device performance.

BACKGROUND

One way to improve transistor performance is through selectiveapplication of stress to the transistor channel region. Stress distorts(i.e., strains) the semiconductor crystal lattice, and the distortion inturn affects the band alignment and charge transport properties of thesemiconductor. By controlling both the magnitude and distribution ofstress in a finished device, manufacturers can increase carrier mobilityand improve device performance. There are several approaches forstraining the transistor channel region. The details concerning theeffects of stress and strain on transistor performance are described ina publication by C. H. Ge et al. in Process-Strained Si (PSS) CMOSTechnology Featuring 3D Strain Engineering, Electron Devices Meeting,Dec. 8-10, 2003, IEDM Technical Digest, IEEE International, whichpublication is incorporated by reference in its entirety.

One conventional approach includes forming an epitaxial, strainedsilicon layer on a relaxed silicon germanium (SiGe) layer. Since SiGehas a larger lattice constant than Si, the epitaxial Si grown on SiGewill have its lattice stretched in the lateral direction, i.e., it willbe under biaxial tensile stress. In this approach, the relaxed SiGebuffer layer is referred to as a stressor that introduces stress in thechannel region. The stressor, in this case, is placed below thetransistor channel region. In another approach, stress in the channelregion is introduced after the transistor is formed. In this approach, ahigh-stress film is formed over a completed transistor. The high-stressfilm distorts the silicon lattice thereby straining the channel region.In this case, the stressor, i.e., the film, is placed above thecompleted transistor structure.

One problem facing CMOS manufacturing is that NMOS and PMOS devicesrequire different types of stress in order to achieve increased carriermobility. For example, a biaxial, tensile stress increases NMOSperformance approximately twofold. However, for a PMOS device, such astress yields almost no improvement. With a PMOS device, a tensilestress improves performance when it's perpendicular to the channel, butit has nearly the opposite effect when it is parallel to the channel.Therefore, when a biaxial, tensile film is applied to a PMOS device, thetwo stress effects almost cancel each other out.

Workers in the art are aware of these problems. Therefore, new CMOSmanufacturing techniques selectively address PMOS and NMOS devices. AnNMOS fabrication method includes using tensile films to improve carriermobility. A PMOS fabrication method includes using substrate structuresthat apply a compression stress to the channel. One PMOS method includesselective application of a SiGe layer into the source/drain regions.Another method uses modified shallow trench isolation (STI) structuresthat compress the PMOS channel.

The use of additional materials, however, adds further processing stepsand complexity to the manufacturing process. Therefore, there remains aneed for improving the carrier mobility of both NMOS and PMOS deviceswithout significantly adding to the cost or complexity of themanufacturing process.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved by preferred embodiments ofthe present invention that provide a device having improved carriermobility and its method of manufacture.

Embodiments of the invention provide a semiconductor fabrication and astructure for strained transistors method. A method comprises forming astressor layer over a MOS transistor. The stressor layer is selectivelyetched over the gate electrode, thereby affecting strain conditionswithin the MOSFET channel region. An NMOS transistor may have a tensilestressor layer, and a PMOS transistor may have compressive stressorlayer. The stressor layer may comprise a material selected from thegroup consisting essentially of a silicon-rich nitride, nitrided siliconoxide (SiON), silicon nitride (SiN, SiN_(x)), and combinations thereof.

Note that although the term layer is used throughout the specificationand in the claims, the resulting features formed using the layer shouldnot be interpreted together as only a continuous or uninterruptedfeature. As will be clear from reading the specification, thesemiconductor layer may be separated into distinct and isolated features(e.g., active regions), some or all of which comprise portions of thesemiconductor layer. In other embodiments, a layer may refer to acontinuous feature having a uniform appearance; yet, it may includeregions having different physical or chemical properties.

Additional features and advantages of embodiments of the invention willbe described hereinafter, which form the subject of the claims of theinvention. It should be appreciated by those skilled in the art that thespecific embodiments disclosed might be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent constructions andvariations on the example embodiments described do not depart from thespirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1-4 are cross-sectional views of the fabrication of a strainedMOSFET according to embodiments of the invention.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the preferredembodiments and are not necessarily drawn to scale. To more clearlyillustrate certain embodiments, a letter or symbol indicating variationsof the same structure, material, or process step may follow a figurenumber.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention. Theintermediated stages of manufacturing a preferred embodiment of thepresent invention are illustrated throughout the various views andillustrative embodiments of the present invention.

This invention relates generally to semiconductor device fabrication andmore particularly to structures and methods for strained transistors.This invention will now be described with respect to preferredembodiments in a specific context, namely the creation of MOS and CMOSdevices. Embodiments of this invention are believed to be particularlyadvantageous when used in this process. It is also believed thatembodiments described herein will benefit other applications notspecifically mentioned. Therefore, the specific embodiments discussed,including exemplary parameter values and ranges of values, are merelyillustrative of specific ways to make and use the invention, and do notlimit the scope of the invention.

Alternative embodiments of the invention are now illustrated within theexemplary context of a conventional MOS transistor 101 such as thatillustrated in FIG. 1. A silicon substrate 105 preferably comprises ap-doped, (100) silicon wafer. The substrate 105 includes an activeregion 110 suitable for forming semiconductor devices. The active region110 may, in this embodiment, comprise a doped well region 110, which isof opposite P or N polarity than the substrate 105. In alternativeembodiments, the channel/substrate orientation may be selected with aview towards optimizing the appropriate charge carrier mobility usingSOI hybrid orientation substrates. For example, a NMOS channel may beoriented along the <100> direction, which is the direction of maximumelectron mobility for a {100} substrate. Alternatively, a PMOS channelmay be oriented along the <110> direction, which is the direction wherehole mobility is maximum for a {110} substrate. The respective devicechannel has a design width from about 0.05 to 10.0 μm, and preferablyless than about 0.5 μm.

An isolation structure, such as a shallow trench isolation (STI) region115, may be formed within the substrate 101 to isolate active region 110from other device fabrication regions in the substrate 105. The STIregions 115 are formed using conventional thermal growth methods andisolation region deposition and patterning methods. Formed over theactive region 110 is a gate dielectric layer 120. The gate dielectric120 may include a thermally grown silicon oxide having a thickness fromabout 5 Å to about 100 Å, and more preferably less than about 20 Å. Inother embodiments, the gate dielectric 120 may include a high-kdielectric having a k-value greater than about 4 and may include, forexample, hafnium-based materials such as HfO₂, HfSiO_(x), HfAlO_(x).Other high-k dielectrics may include Ta₂O₅, TiO₂, Al₂O₃, ZrO₂, HfO₂,Y₂O₃, L₂O₃, and their aluminates and silicates.

A gate electrode 125 is formed over the gate dielectric 120 layer. Thegate electrode 125 may comprise metals, metal alloys, metal-containingmaterials, polysilicon, polysilicon, and polycide (dopedpolysilicon/metal silicide stack) gate electrode materials. Preferably,the gate electrode 125 comprises chemical vapor deposition (CVD)polysilicon between about 100 Å and about 10,000 Å thick and morepreferably between about 500 Å and about 2,000 Å. The gate electrode 125may further include about 1E20 cm⁻³ dopant of polarity opposite thechannel region of the corresponding MOS device to be formed therefrom.Such doping advantageously provides for enhanced off current (Ioff)performance, enhanced drain saturation current (Idsat) performance andpossibly enhanced short channel effect (SCE) performance of the PMOSdevice.

An optional glue layer (not illustrated) maybe is formed between thegate dielectric layer 120 and the gate electrode 125. The glue layerpromotes adhesion between adjacent layers. It may formed by CVD of polysilicon, amorphous silicon, TiN, Ti, Ta, TaN, or combinations thereof.

Using the gate electrodes 125 as a mask, lightly doped source/drain(LDS/LDD) regions 130 are formed in the substrate 105 to a depth betweenabout 100 Å and about 1000 Å and preferably between about 200 Å andabout 400 Å. An LDS/LDD region 130 is formed by ion implanting a dopantsuch as boron or phosphorous. After annealing the concentration ofphosphorus or arsenic dopant in the LDS/LDD regions 130 is preferablybetween about 5E16 atoms/cm³ to about 1E19 atoms/cm³.

Between the LDS/LDD regions 130 and under the gate electrode there is achannel region 135. Formed on sidewalls of the gate electrode 125 aresidewall spacers 140. The sidewall spacers 140 are a dielectric, such asCVD silicon oxide. Using the gate electrodes 125 and also sidewallspacers 140 as a mask, heavily doped source/drain 145 regions areformed. The source/drain regions 145 may extend below the LDS/LDDregions 130. After annealing, the concentration of dopant in the regions145 is preferably between about 5E18 atoms/cm³ and about 5E20 atoms/cm³.The sidewall spacers 140 may comprise a composite structure formed ofseveral layers.

Turning now to FIG. 2 there is illustrated the MOS transistor 101 ofFIG. 1, wherein the MOS transistor 101 further comprises a stressorlayer according to an embodiment of the invention. The stressor layer205 may comprise strain-inducing layer suitable for used in advancedsemiconductor devices. The stressor layer 205 is preferably about 200 Åto about 1000 Å thick. The stressor layer 205 comprises a tensile stresslayer, although in other embodiments it comprises a compressive stresslayer. A process used to form the stress layer 130 may include plasmaenhanced chemical vapor deposition (PECVD), low pressure chemical vapordeposition (LPCVD), atomic layer deposition (ALD), rapid thermalchemical vapor deposition (RTCVD), physical vapor deposition (PVD),individually or in combination. The stressor layer 205 may comprise asingle layer or a plurality of layers.

In an embodiment, the stressor layer 205 comprises a contact etch stoplayer, such as silicon nitride. Stoichiometric silicon nitride films areknown to be highly tensile stressed on silicon. However, the tensilestress may be greatly lowered and even turned into compressive stress byadjusting the Si/N ratio. Generally, adding more silicon makes thesilicon nitride film more compressive, while adding more nitrogen makesit more tensile. For example, the intrinsic stress of silicon nitride onsilicon is preferably adjusted from about 300 MPa to about 1700 MPa byadjusting the Si/N ratio. Stress levels between about −5.0 GPa to about+5.0 GPa, and beyond, are within the scope of embodiments of theinvention. The stressor layer thickness may be between about 5 nm and500 nm.

The stressor layer 205 when compressive is preferably comprised ofsilicon nitride (Si₃N₄ or SiN_(x)), silicon oxynitride (SiON), oxide, aSi-rich nitride, or a N-rich nitride. The compressive stressor layer 205is more preferably SiN or SiON and is most preferably SiON. It has athickness from about 200 Å to about 1000 Å, and preferably from about250 Å to about 500 Å. The stressor layer 205 is preferably deposited byplasma enhanced chemical vapor deposition (PECVD). PECVD conditionsinclude a temperature about 300° C. to about 600° C. Deposition time isabout 10 seconds to about 500 seconds and preferably from about 20seconds to about 120 seconds. The reactant NH₃:SiH₄ gas ratio is about4:1 to about 10:1, and preferably less than about 8:1. Alternativereactants include a di-saline:NH₃ gas ratio from about 1:4 to about1:10, and preferably less than about 1:1. The deposition pressure ispreferably about 1.0 Torr to about 1.5 Torr. The PECVD power used toform the compressive stressor layer 205 is preferably from about 1000 Wto 2000 W and more preferably greater than about 1000 W.

In alternative embodiments, the stressor layer 205 is a tensile stressorlayer 205. When the stressor layer 205 is a tensile stress layer,suitable materials include, silicon nitride, tetraethylorthosilicate(TEOS), silicon oxynitride (SiON), oxide, Si-rich nitride, or a N-richnitride, and it is preferably SiN or SiON. The tensile stressor layer205 has a thickness from about 200 Å to about 1000 Å, and preferablyfrom about 250 to about 500 Å. The tensile stressor layer 205 ispreferably deposited by rapid thermal chemical vapor deposition (RTCVD).The RTCVD temperature is 350° C. to about 800° C., and preferably fromabout 400° C. to about 700° C. Reaction time is about 10 seconds toabout 2000 seconds, and preferably about 20 seconds to about 120seconds. The NH3:SiH4 gas ratio is about 50:1 to about 400:1, andpreferably less than about 700:1. An alternative reactant compositionincludes a di-saline:NH3 gas ratio about 1:40 to about 1:500, andpreferably less than about 1:1. The deposition pressure is preferablyabout 10 Torr to about 400 Torr, preferably less than about 300 Torr.

Continuing with FIG. 2, a masking layer 210 is formed at least over aportion of the MOS device 101. Masking layer 210 may include aphotoresist or a hardmask. Suitable hardmasks include oxides, nitrides,oxynitrides, or silicon carbide, for example. As illustrated in FIG. 2,masking layer 210 is selectively formed over the MOS device 101, andthen planarized. Planarizing may be performed using conventionalchemical mechanical polishing (CMP) methods. A resist mask may furtherinclude a re-flow to enhance the planarization if necessary. Thestressor layer 205 may function as a polish stop layer.

The stressor layer 205 as shown in FIG. 2 is known to induce a stress inthe channel between 135 the source/drain regions 145 of a MOS device101. For example, a highly tensile stress/strain film is known to havenet tensile stress/strain effect upon the channel region 135. Likewise,a highly compressive stress/strain film is known to induce a compressivechannel stress/strain with the channel region 135 of a MOS transistor101 such as illustrated in FIG. 3.

One problem with a blanket deposition of a stressor layer 205 over a MOSdevice is that different regions of the stressor layer have differenteffects upon the channel region. For example, that portion of thetensile stressor layer 205 over the source/drain regions 145 exerts alarge tensile force on the channel region 135. In contrast, that portionof the stressor layer 205 over the gate electrode 125 exerts relativelyweaker compressive force on the channel region 135. Therefore, theimproved MOSFET performance gained from one region of the stressor layeris partially degraded by a competing region of the stressor layer.

Turning now to FIG. 3, the above discussed problems with stressengineering are addressed by completely removing the stressor layer 205is removed from over the gate electrode 125 the MOS device 101. Althoughit is well known in the prior art to form an opening in a stressor layerover the gate electrode such opening—referred to as contact openings orcontacts—are relatively small and do not impact the overall stressapplied by the stressor layer. By contrast, it has been found thatcompletely removing stressor layer 205 from over the entire region ofgate electrode 125 can have a beneficial impact on the stress applied tochannel region 135. Removal can be accomplished by either wet etching ordry etching. Preferably, the removed portion 305 comprises at least thatportion of the stressor layer 205 over the gate electrode 125. In otherembodiments, the removed portion 305 comprises further comprises some ofthe stressor layer 205 over the sidewall spacers 140. In otherembodiments (not shown), the etching completely removes the stressorlayer 205 from over the sidewall spacers 140, so that essentially noneof the stressor layer 205 contacts the spacers 140.

The mask 210 is next removed, thereby producing the structureillustrated in FIG. 4. In an embodiment of the invention, the MOS device101 is an NMOS transistor, and the stressor layer 205 is a tensilestressor layer. In another embodiment, the MOS device 101 is a PMOStransistor, and the stressor layer 205 is a compressive stressor layer.The combinability of embodiments of the invention makes them suitablefor CMOS device fabrication, since NMOS and PMOS devices are selectivelytreated. In embodiments comprising both NMOS and PMOS devices, devicesof opposite polarity may be adjacent or located in different regions ofthe substrate. After this, the device may be completed (silicidecontacts, metallization, ILD layers, etc.) using conventional methods.

The semiconductor device of FIG. 4 is provided to illustrate a usefulapplication of an embodiment of the invention. However, otherapplications are within the scope of embodiments of the invention.Embodiments are not limited to transistors as the invention relates moregenerally to forming strained semiconductor devices. For example, anembodiment provides a method of fabricating a strained channeltransistor. The method comprises forming a gate electrode over asubstrate. The substrate may comprise a semiconductor crystal, whereinan interatomic distance between neighboring atoms in the semiconductorcrystal is defined by a substrate lattice spacing. The method furtherincludes adjusting the substrate lattice spacing under the gateelectrode. For a silicon substrate, the substrate lattice spacing isabout 5.4 Å (silicon=5.4295 Å) at about 25° C. Adjusting the latticespacing may induce a strain under the gate electrode, i.e. the carrierchannel, by at least about 0.1% or about 0.0054 Å. The adjustingpreferably comprises forming strained layer over the gate electrode andover the substrate, and removing a portion of the strained layer fromover the gate electrode.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method of forming a semiconductor device, the method comprising:forming a MOS device in a substrate, the MOS device comprising a source,a drain, a channel region between the source and the drain, a gateelectrode over the channel region, and a pair of sidewall spacesadjacent opposite sides of the gate electrode; forming a stressor layerover the MOS device; and substantially completely removing the stresslayer from over the gate electrode.
 2. The method of claim 1, whereinthe substrate comprises a material selected from the group consistingessentially of silicon, silicon germanium, or combinations thereof. 3.The method of claim 1, wherein the stressor layer comprises one or morelayers, at least one of said one or more layers comprising a materialselected from the group consisting essentially of a silicon-richnitride, nitrided silicon oxide (SiON), silicon nitride, combinationsthereof.
 4. The method of claim 1 wherein no portion of the stressorlayer contacts a portion of the gate electrode.
 5. The method of claim1, wherein the stress layer is about 200 to 1000 Å thick.
 6. The methodof claim 1, further comprising selectively removing a portion of thestress layer from over the sidewall spacers.
 7. The method of claim 1,further comprising forming a masking layer over the stressor layer, andplanarizing the masking layer and the stressor layer so that the layersare coplanar.
 8. The method of claim 1 wherein substantially completelyremoving the stressor layer comprises etching the stressor layer overthe gate electrode.
 9. A method of forming a semiconductor device, themethod comprising: forming a gate electrode having a pair of sidewallspacers over a substrate; forming a stress layer over the gateelectrode, the sidewall spacers, and the substrate, thereby forming anintermediate structure; planarizing the intermediate structure; andetching the stressor layer from over the gate electrode afterplanarizing the intermediate structure, such that no portion of thestressor layer remains over the gate electrode.
 10. The method of claim9, wherein planarizing the intermediate structure includes using thestress layer as a polish stop layer.
 11. The method of claim 9, whereinthe stress layer comprises a material selected from the group consistingessentially of a silicon-rich nitride, nitrided silicon oxide (SiON),silicon nitride, and combinations thereof.
 12. The method of claim 9,wherein the stress layer is about 200 to 1000 Å thick.
 13. The method ofclaim 9, wherein an intrinsic stress of the stress layer is at leastabout 500 MPa.
 14. The method of claim 9 further comprising etching thestress layer from over at least a portion of the sidewall spacers. 15.The method of claim 9 further comprising etching the stress layer fromover the entire sidewall spacers.
 16. A method of fabricating a strainedchannel transistor, the method comprising: forming a gate electrode overa substrate, the substrate comprising a semiconductor crystal, whereinan interatomic distance between neighboring atoms in the semiconductorcrystal is defined by a substrate lattice spacing; and adjusting thesubstrate lattice spacing under the gate electrode, the adjustingcomprising: forming a strained layer over the gate electrode and overthe substrate, and removing the strained layer from over the gateelectrode.
 17. The method of claim 16, wherein the substrate latticespacing under the gate electrode is adjusted at least 0.10%.
 18. Themethod of claim 16, wherein the strained layer comprises a materialselected from the group consisting essentially of germanium, carbon,silicon, silicon germanium, a carbide, a nitride, and combinationsthereof.
 19. The method of claim 16, wherein the substrate latticespacing is about 5.4 Å at about 25° C.